The invention relates to a hydraulic dual-circuit braking system for a road vehicle having front wheel and rear wheel brakes which are controlled by a front axle and a rear axle braking circuit means I and II.
A supply of braking pressure is provided which is provided by pedal force and, if necessary, provided using a braking force amplifier. Each of the braking circuits includes a static main cylinder, each arranged adjacent to one another in a twin-type of construction in a common housing. The pistons of each cylinder are mounted for displacement by one arm of a rocker displaced by the possibly amplified pedal force. The rocker is pivotable about an axis which extends vertically with respect to this direction of displacement of the pistons, and has rocker arms L.sub.V and L.sub.H, the ratio of which corresponds to the ratio of forces applied to the front and rear brakes. The pedal forces provide for a dynamically stable braking behavior of the vehicle in an entire range of vehicle decelerations.
The braking system also is provided with an electronic anti-blocking system (ABS) which, in response to the position of the front and rear main brake piston, provides a blocking of the pressure actuating force to the front and rear brakes. Such a braking system is the subject matter of German Unexamined Patent Application No. P 37 00 697.5-21.
In the braking device, the ratio L.sub.V /L.sub.H of the effective lengths L.sub.V and L.sub.H of the rocker arms, which are supported at one end of the pistons of the front axle main cylinder and of the rear axle main cylinder, respectively, are equal to the ratio F.sub.H /F.sub.V of the effective piston areas F.sub.H and F.sub.V of the respective main cylinder pistons. As a result, identical displacement distances of the pistons, in the sense of a pressure build-up, are also linked to identical outlet pressures in the output pressure spaces of the two main cylinders. The resultant installed braking force distribution corresponds to a permanent balancing of the braking force ratio F.sub.VA /F.sub.HA.This permanent balancing of the installed braking force distribution is arranged in such a manner that a stable dynamic braking behavior of the vehicle is obtained in the entire range of variations of the possible braking forces of vehicle decelerations. This design of the braking device enables the latter to be utilized in a simple manner with an anti-blocking system, which operates in accordance with the principle of volume expansion of the output pressure spaces of the main cylinders.
Although this design of the braking system is very advantageous: with respect to the implementation of the anti-blocking control functions; with respect to the safeguarding against a failure of one of the braking circuits, in which case the other braking circuit can still be braked with braking force amplification; or with disturbance of the braking force amplifier, in which case adequate braking deceleration can be achieved via both braking circuits with the pedal force alone, it has the disadvantage that, because of the permanent balancing of the brake force distribution in its entire range of variation, the rear wheel brakes make only a relatively small contribution to the total braking force, compared with an ideal braking force distribution, in the part-braking range. The consequence of this is that the front wheel brakes, because they have to generate the predominating part of the braking force utilized, must be dimensioned to be very much stronger than the rear wheel brakes, which creates considerable problems with respect to the constructional size of the front wheel brakes, particularly in high-power vehicles. Furthermore, the front wheel brakes are also subject to much greater wear than the rear wheel brakes. In the part braking range of interest, an increased contrition of the rear wheel brakes to the braking force ca be utilized when the vehicle is equipped with an anti-blocking system, by providing a brake pressure control, and utilizing functional elements of the anti-blocking system to such an extent that the rear wheel brake slip is adjusted to a value which, as long as the anti-blocking system has not responded, is always slightly greater than the front wheel brake slip. However, a very accurate monitoring of the brake slip, both of the front wheels and of the rear wheels, is required which, in order to be implemented, requires severe changes in an electronic control unit of the ABS which is also utilized for controlling the braking pressure distribution, thus requiring extensive technical alterations. An additional problem is that a braking force distribution control system operating in accordance with the principle of brake slip monitoring ASB in the above sense, operates with adequate accuracy only above a minimum speed of the vehicle which is about 30 km/h.
It is therefore the object of the present invention to improve a braking system of the type initially mentioned, by means of advantageously little technical expenditure, a front axle/rear axle braking force distribution can also be achieved with a braking system not subject to anti-blocking control, which enables an increased rear axle braking force component to be utilized in the part braking range without a loss of dynamic stability of the vehicle.
According to the present invention, this object is achieved by having position transmitters to indicate the positions of the front and rear main brake cylinders and to input these into an electronic control. The control calculates an ideal front and rear wheel braking force ratio (F.sub.V /F.sub.H), correlated to a particularly desired deceleration, and compares this to the actual called for braking forces. When the value of the actual rear braking force is lower by more than a threshold value F.sub.HA1 than the ideal value F.sub.HAi, the electronic control blocks any actuating pressure change to the front brake from the front main cylinder, while allowing increases to the actuating pressure for the rear brakes, until the rear brake force approaches the ideal force by a second and lesser threshold value F.sub.HA2, at which time the blocked pressure at the front brakes is released to allow both brake actuating pressure forces to respond to the brake pedal input.
Accordingly, each of the two main cylinders of the braking device is provided with an electric inductive position transmitter which emits an electric output signal characteristic of the respective piston position. Because of the relationship between piston position and braking pressure, which, although it is non-linear in the range of relatively low braking pressures and overall is monotonic and thus unambiguous, these signals are also a measure of the actuating braking pressure, and thus also a measure of the front axle braking force F.sub.VA and the rear axle braking force F.sub.HA. They are supplied as information inputs to a processing unit which detects from them the current braking force of the desired deceleration force distribution. This processing unit compares the actual called for rear axle braking force component with the ideal value, referred to the respective front axle braking force component, which ideal value corresponds to equal force closure utilization at the front axle and the rear axle brakes. If this comparison shows that the current value of the rear axle braking force component, determined by means of the position transmitter of the rear axle main cylinder, is less by more than a difference threshold .DELTA.F.sub.HA1 of, for example, 15% to 20% than the ideal value F.sub.HAi', the processing unit generates a signal which in turn triggers the actuation of the braking pressure control valves of the front wheel brakes to their blocking Position. After that, an increase in operating force initially only causes the rear axle braking force component to be increased as a result of which the braking force distribution develops to the ideal value F.sub.HAi of the rear axle braking force logically combined with the current value of the front axle braking force component. If, during this process, the rear axle braking force component reaches a value which is less by less than a second difference threshold .DELTA.F.sub.HA2, the amount of which is less than the first-mentioned threshold and which is about 5%, than the ideal value F.sub.HAi, the processing unit generates a signal which triggers the switching back of the braking pressure control valves of the front axle braking circuit into their basic open position. Due to the automatically controlled repetition of such braking force distribution control cycles, a step or sawtooth-shaped approximation of the actually utilized braking force distribution to the ideal braking force distribution is achieved. The rear axle braking force component being less by at least the difference amount .DELTA.F.sub.HA2 and at the most by the difference amount .DELTA.F.sub.HA1 than the ideal value in each case. The front wheel brakes are less loaded because of the more effective utilization of the rear wheel brakes which benefits both in a reduction in wear and in operating reliability. On the other hand, the front wheel brakes can be dimensioned to be slightly smaller, compared with a vehicle without the braking force distribution control which has constructional advantages with respect to the accommodation of the front wheel brakes.
If the relationship between piston position and initial pressure of the main cylinders is known, it is possible to calibrate the output signals of both position transmitters in a simple manner in units of pressure, and also the respective braking force component and the venting condition can also be separately checked for each of the two braking circuits. This calibration is obtained with the vehicle being stationary and the operating brake pressure exceeding a threshold value P.sub.O, wherein a comparison of the actual pressure of the front and rear main brake cylinders with respect to these sensed positions are calculated.
If the displacement distance, detected by means of the respective position transmitter, of a piston to the response of the pressure switch is only slightly greater than the minimum value logically combined with the optimum venting condition of the braking system, this difference is taken into consideration in the sense of a correction by the processing unit In principle, this relies on the fact that, from the difference between the measured and the ideal value, a correction factor is formed which corresponds to the ideal value/measurement value ratio, so that the ideal value can be used for further processing. The correction is achieved by multiplying the measured value by the correction value. If the measured displacement distance of the respective main cylinder piston is large (greater by more than a difference amount which forms a tolerance threshold than the ideal value), this is an indication of the fact that the degree of venting in the respective braking circuit is too defective, and that the braking system must undergo maintenance. This is indicated to the driver by means of a visual and/or audible warning signal. The permanently balanced braking force distribution is detected an checked by measuring the piston positions or braked pressures, respectively, in the output pressure spaces of the main cylinders with a predetermined pressure P.sub.O in a braking force distribution diagram.
Since the ideal braking force distribution in a vehicle is not only dependent on the vehicle geometry but also on the loading condition of the vehicle, particularly on the load distribution in the vehicle, it is advantageous if the actual braking force components are taken into consideration in the calculation of the ideal braking force distribution in order to be able to achieve a most effective utilization of the rear axle braking force component by means of the braking force distribution control system according to the invention.
Also, it is desirable if the threshold value is lowered in accordance with respect to transverse forces acting on the vehicle which, in practice, lead to different ideal braking force distributions having to be stipulated for the lefthand vehicle side and the righthand vehicle side.
Vehicle loading and weight distribution of the vehicle can be obtained in response to the static load condition of the vehicle spring suspension, and this can be used for determining the total vehicle weight and the weight distribution of the vehicle. Such sensors can be constructed, for example, as spring travel transmitters which enable the wheel loads to be detected or also as inclination transmitters which respond to a longitudinal or transverse inclination of the vehicle.
As an alternative to this or in combination with this, the total vehicle weight and axle load distribution can also be determined by utilizing the operating data of the vehicle, namely: engine speed, throttle value position and gear transmission for transmitting torque (sensed in the drive shaft of the vehicle). Alternatively, changes in vehicle acceleration and braking forces causing the same can be utilized. In this connection, the measuring accuracy can be considerably increased by forming a mean value over the results of several individual dynamic measuring processes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.